A multi-step electrophotographic (EP) process is the basis for numerous laser printers, copiers, multiple function devices, and other configurations. In a first step, a photoconductor is electrically charged for the subsequent formation of latent images. The photoconductor may be charged using one or more charge roller, corona or scorotron in some implementations. In but one example, the Indigo 3000 press available from The Hewlett-Packard Company has three sets of double scorotrons for charging the photoconductor.
A charge roller may include a metal shaft with a conductive elastomer surrounding it. The outer portion has been constructed in two ways in some arrangements. A first example includes a single layer with a moderately conductive material, usually an ionic conductive agent, mixed inside. A single-layer charge roller may also have a thin (e.g., thickness of a few microns) insulating layer outwardly of the conductive material or layer.
Another example of a charge roller has plural layers including an insulating outer sleeve of increased thickness (e.g., greater than a few microns) and an inner elastomeric region which may be loaded with a highly conductive network such as carbon. A double-layer charge roller generally charges less uniformly compared with a single layer charge roller due to the difficulty in providing a constant sleeve thickness. Accordingly, a single-layer charge roller system may provide images of increased quality and may be preferred for high-quality color image applications.
A core of a charge roller may be supplied with direct current (DC) electrical energy and possibly an additional alternating current (AC) voltage during use. If DC energy is used alone, the shaft voltage may be roughly 600 V higher than a desired voltage to be provided at a surface of the photoconductor. The extra 600 V is provided to generate ions in the air as dictated by the Paschen curve. With usage of AC energy, the voltage of the DC energy may be close to a desired photoconductor voltage with an AC amplitude of 600 V peak or more. The addition of AC usually creates a more uniform charge layer on the photoconductor adding or subtracting the photoconductor surface charge as needed.
The conduction mechanism of a single-layer charge roller is mobile ion movement in response to an applied electric field. If material (e.g., elastomer) is sandwiched between the two electrodes (e.g., charge roller shaft and photoconductor) and a voltage is applied, a current flows, generally falling with time. This is consistent with ions moving and accumulating at one side and leaving behind a charged layer of the opposite polarity on the other side which decreases the electric field available for moving current within the layer. Some charge injection may also occur at the electrodes which could neutralize some of the ions, thus decreasing the ion concentration over time.
For printers of relatively reduced speed, the charge roller may be engineered to last the life of a replaceable cartridge. For relatively high-speed machines, the charge roller may be expected to have extended use before needing replacement.
Over time, a charge voltage of a photoconductor may fall or drop. For example, referring to FIG. 1, measurements indicate that photoconductor charge voltage may drop over time with partial recovery during stoppage of imaging operations. In some situations, recovery may not occur even after imaging operations have ceased (e.g., no recovery is evident after a hour break at approximately 80,000 photoconductor page cycles (impressions) of FIG. 2).
The non-recovery of FIG. 2 may probably be attributed to two factors. First, a charge roller surface having a charge may attract charges of the opposite sign during operation. This can be partially remedied by turning off the charge roller since the charges may turn back towards the conductive shaft. In addition, some ions may leach from the surface due to relatively high concentrations and proximity to the surface and are lost due to recombination, decreasing the roller ion concentration. The long-term reduction in the photoconductor charge voltage is likely due to the latter. Accordingly, DC bias may be increased to provide a substantially constant photoconductor charge voltage.
The above-described situation may worsen if high-speed printing is implemented in non-stop applications because the charge roller may not be allowed to sufficiently recover. In this case, additional problems may be present. For example, the charge roller may physically degrade due to high ionic concentration at an interface for most desired charge roller formulations. The high concentration may alter a local environment within the charge roller causing polymeric bonds of the charge roller to break. The result is that the elastomeric portions of the charge roller may return to a liquid state in the local region. The environment may be catalytic because the deterioration has been observed to continue long after the voltage is removed and the charge roller may continue to disintegrate. Affected regions of the charge roller may have a surface defect or a sticky surface stain which may negatively impact electrophotographic processes.
Rollers of similar composition used to donate charge or bias a surface may suffer the same drawbacks (e.g., if run at high speed in non-stop applications). An example is a transfer roller used in some electrophotographic applications which helps move toner from the photoconductor to the printing media wherein a voltage is applied between the transfer roller and the photoconductor to attract toner from the imaged photoconductor.
At least some aspects of the disclosure include methods and apparatus for providing improved generation of hard images upon media.